1. Field of the Invention
The invention pertains to an axial friction bearing for supporting the rotating shaft of an exhaust gas turbocharger connected to a lubricating oil circuit.
2. Description of the Related Art
Hydrodynamic axial thrust bearings are used in many cases to support exhaust gas turbochargers, that is, as plain bearings for the rotating shafts to absorb axial forces and to provide guidance in the axial direction.
Especially in the case of axial-flow turbochargers, the flow conditions produce high axial thrust, which is usually transmitted to the housing by a hydrodynamic axial thrust bearing provided with profiled bearing surfaces.
For example, U.S. Pat No. 6,024,495 discloses an axial friction bearing of this type which includes a bearing body, permanently connected to a bearing housing; a bearing comb, which rotates along with the shaft; and at least one lubricating gap, provided between the bearing box and the bearing comb. The gap is formed between the profiled surface of a ring and a flat sliding surface and is connected to an oil supply. A shoulder of the shaft, that is, the bearing comb seated on the shaft, therefore runs at least indirectly against an end surface of the stationary bearing housing. The surface of the ring has several radially oriented longitudinal oil grooves.
Dirt particles, however, are usually present in the lubricating oil, and the size of these particles is on the same order of magnitude as the thickness of the lubricating gap. The damaging effect of the dirt particles increases as the thickness of the lubricating gap decreases.
Various solutions have already been proposed to counteract this effect.
For example, according to U.S. Pat No. 6,024,495, axial friction bearings of this type are provided with a convergent lubricating gap, which is advantageous for developing hydrodynamic pressure, by machining wedge surfaces oriented in the circumferential direction into bearings in such a way that the lubricating gap is smallest in the area of the adjacent lubricating oil groove.
So that the flow-induced axial forces acting on the rotor can be absorbed in the case of axial thrust bearings with permanently machined-in wedge surfaces, a trap surface, across which a lubricating gap outlet extends, is provided next to each of the wedge surfaces.
Conformal surfaces are characteristic of hydrodynamic lubrication. A load-supporting pressure develops in the lubricating film when the lubricating gap narrows down, when a viscous lubricant is used, and when a sliding movement in the direction of the narrowing of the gap occurs. If sufficient lubricant is drawn into the convergent lubricating gap, the surfaces are completely separated by the lubricant, which is crucial to the prevention of wear and thus to the reliability of operation.
So that the requirements for low thrustal losses and acceptable oil throughputs can also be fulfilled, solutions are known which use a free-floating axial bearing disk between the rotor and the housing. In the design of an axial bearing with a free-floating disk, the oil has a high volume flow rate, because there are two lubricating gaps and because the centrifugal effect causes the oil to flow away across the wedge surface. This has the effect of decreasing the amount of pressure which can be built up along the wedge surface and of reducing the thickness of the lubricating gap.
Thus a rigid axial bearing unit already tested by the applicant comprises a fixed-segment bearing with lubricating grooves, each of which is bounded on the outside by a sealing web, through each of which a radially outward-leading dirt and cooling groove passes. These bearings have a good load capacity, but because of the high shear rate, they also have high thrust losses and a certain tendency to fail “spontaneously” (see below).
An axial bearing with a free-floating disk, which rotates around or with the shaft, is known especially from U.S. Pat No. 6,024,495, which is incorporated herein by reference. Here the wedge surface is bordered radially by an outer sealing web, and the sealing web is interrupted by a radially outward-leading dirt and/or cooling groove. The goal of this sealing web, which borders both the lubricating oil grooves and the wedge surfaces, is to decrease the amount of lubricating oil that can escape radially to the outside in the area of the wedge surfaces. This solution, too, suffers from the disadvantage that a design of this type also tends to fail spontaneously (see below).
The mechanism of spontaneous failure operates as follows: The lubricating gap has its minimum height at the sealing web. A high axial force requires the (hydrodynamic) buildup of greater compressive forces; greater compressive forces require a narrower lubricating gap; and a narrower lubricating gap or a smaller gap height at the sealing web decreases the volume flow rate of the oil. A decrease in the volume flow rate of the oil, however, leads to reduced cooling and to a higher average oil temperature; a higher average oil temperature leads to a sharp drop in the viscosity of the oil; and a sharp drop in the viscosity of the oil prevents in turn the buildup of hydrodynamic compressive forces. This effect is dominant over that by which a narrower lubricating gap causes higher compressive forces. The result is spontaneous failure, that is, the sudden collapse of the hydrodynamic compressive forces and the occurrence of mechanical contact between the thrust surfaces.